The present invention relates generally to low noise amplifiers, and more particularly to low noise amplifiers having programmable gain.
By way of background, resistance in series between the source electrodes of a pair of differentially coupled MOSFETs is the main source of noise in a differential amplifier. The prior art circuit shown in FIG. 1 illustrates a very low noise differential amplifier 1 which includes a P-channel current source transistor Q1 having its drain connected directly by a conductor 2 to the sources of a pair of P-channel input transistors Q2 and Q3. The drains of input transistors Q2 and Q3 are connected by output conductors 3 and 4 to load resistors 5, and 6, which have a resistance RL. Input voltages Vin+ and Vin− are applied to the gates of input transistors Q2 and Q3, respectively, and output voltages Vout− and Vout+ are produced on output conductors 3 and 4, respectively. The reason that the noise of amplifier 1 is very low is that there is no resistor connected between the source electrodes of transistors Q1 and Q2.
The gain of amplifier 1 is given byGain=Gm×RL,  Equation (1)where Gm is the transconductance of the pair of differentially connected input transistors Q2 and Q3 and is given byGm=SQR{μ·Cox·(W/L)·I},  Equation (2)where SQR means the square root of the bracketed expression, μ is the mobility of majority carriers in the channel regions of input transistors Q2 and Q3, Cox is the capacitance formed by the gate and channel region of each of the input transistors Q2 and Q3, W/L is the channel-width-to-channel-length ratio of each of input transistors Q2 and Q3, and I is the current flowing through each of input transistors Q2 and Q3.
Prior art low noise amplifier 1 is somewhat non-linear and introduces distortion, which, however, is acceptable in some applications. If the distortion is not acceptable, it can be reduced in various ways, as explained in commonly owned U.S. Pat. No. 6,118,340 issued Sep. 12, 2000 to present inventor Koen.
Usually, the tail current I0 in current source transistor Q1 is constant, in order to enable amplifier 1 to achieve a particular output voltage swing. Consequently, the gain of prior art low noise amplifier 1 is not variable or programmable because the ratio W/L is fixed. However, there are many applications in which programmable gain of an amplifier is very desirable.
For systems that must process signals with an extremely wide dynamic range, such as ultrasound, with low distortion, it is often necessary to lower the gain of the input to the low noise amplifier. Being able to lower the amplifier gain by electronic means is also very desirable, as parasitic impedances are often developed when electrodes that might be used to adjust the gain are brought out to the edge of a semiconductor package. These parasitic impedances often cause the gain to be imprecise and can lead to circuit oscillation. By being able to lower the amplifier gain, a larger amplitude input signal can be handled, compared to the case where the amplifier is configured to have only high gain. The high gain setting is desirable to achieve the lowest noise but will result in “overload” from strong input signals. Allowing the gain to be reduced results in higher noise, but the amplifier can handle a larger amplitude input signal.
There are various known techniques for providing programmable gain amplifiers, some of which involve switching of gain resistors of various values into and/or out of connection between the sources of input transistors such as Q2 and Q3. Other prior programmable gain amplifiers utilize various techniques to controllably adjust or vary the resistance of gain resistors coupled between the sources of input transistors such as Q2 and Q3. Unfortunately, any resistance connected between the sources of input transistors Q2 and Q3 introduces noise, and therefore the prior programmable gain amplifiers of this kind inherently have relatively high noise compared to the noise of the basic prior art low noise amplifier 1 of FIG. 1. (The noise introduced by a resistance R coupled between the sources of differentially coupled input transistors such as Q2 and Q3 is approximately equal to the square root of 4·k·T·B·R, where k is Boltzmann's constant, T is the absolute temperature in degrees Kelvin, and B is the bandwidth.)
The prior techniques of using external switches to selectively switch external resistors so as to control the gain of an amplifier is costly and inconvenient because of the physical size, and also reduces circuit performance because of parasitic capacitances associated with the external resistor and switch.
Thus, there is an unmet need for a low noise programmable gain MOS amplifier.
There also is an unmet need for a low noise, programmable gain MOS or CMOS amplifier that can extend the dynamic range of signals which can be amplified with low distortion.
There also is an unmet need for a low noise, programmable gain MOS or CMOS amplifier that can extend the dynamic range of input signals which can be amplified with low distortion.
There also is an unmet need for a low noise, programmable gain CMOS or MOS amplifier which avoids the need for and costs associated with use of external gain control resistors and large amount of area required for the external gain control resistors on a printed circuit board.
There also is an unmet need for a way of controlling the gain of programmable gain amplifier with a minimum number of gain control signal conductors.